Magnesium is a lightweight and provides rich resources, and thus, magnesium is specifically noted as a material for weight reduction for electronic devices, structural members, etc.
On the other hand, in order to apply to the structural parts, i.e., rail ways and auto mobiles, the alloy needs to show the high strength, ductility and toughness, from the viewpoints of safety and reliability for the human been.
FIG. 1 shows a relationship between the strength and the elongation-to-failure of wrought magnesium alloys and cast magnesium alloys; and FIG. 2 shows a relationship between the specific strength (=yield stress/density) and the fracture toughness. It is known that wrought alloys show higher ductility and toughness than those of the casted alloys. Therefore, the wrought process, i.e., strain working, is found to be one of the effective methods to obtain excellent characteristics of strength, ductility and toughness.
However, when magnesium alloys are produced by wrought process through rolling, extrusion, there is a problem that the alloy has a strong texture due to the process. Therefore, a conventional wrought magnesium alloy could have a high tensile strength at room temperature; however this alloy shows a low compression strength. Accordingly, when a conventional wrought magnesium alloy is applied to mobile structural parts, there is a large defect; the part, which is applied the compressive strain, occurs brittle fracture and the lacks of isotropic deformation.
Recently, it has been found that the formation of a specific phase, i.e., quasi-crystal phase, which possesses five-fold symmetry and is very different from crystalline phases, has discovered in an Mg—Zn-RE alloy (where RE=Y, Gd, Dy, Ho, Er, Tb).
The quasi-crystal phase has a good matching to a magnesium matrix interface, i.e., the interface between magnesium and quasi-crystal phase is coherency. Therefore, the dispersion of a quasi-crystal phase in a magnesium matrix causes to the reduction of the basal texture and can enhance the compression strength with high tensile strength. In addition, this alloy can reduce the yield anisotropy, which is an unfavorable characteristic to apply the structural parts.
However, in order to form a quasi-crystal phase in a magnesium alloy, there is a serious problem that the addition of a rare earth element is indispensable. The rare earth element is an element that is rare and valuable. Therefore, if the alloy with the addition of rare earth elements could exhibit good properties, its material cost is expensive; not advantage from the industrial point of views.
Concretely, Patent References 1 to 3 merely specify that, the addition of a rare earth element (especially yttrium) is necessary to form the quasi-crystal phase in magnesium.
Patent Reference 4 merely shows that, the addition of yttrium and other rare earth element is indispensable to form the quasi-crystal phase in magnesium. The problem that the wrought magnesium alloy shows the yield anisotropy, could be solved due to the dispersion of quasi-crystal phase and the grain refinement.
Patent Reference 5 merely specifies that the addition of yttrium and other rare earth element is indispensable to form the quasi-crystal phase in magnesium. This reference shows the working conditions (working temperature, speed, etc.) at the secondary forming using the magnesium alloys with dispersion of quasi-crystal phase.
Non-Patent References 1 and 2 describe the formation of a quasi-crystal phase of Mg—Zn—Al alloy. However, since the phase is a quasi-crystal single phase, an Mg matrix does not exist in this alloy.
In Non-Patent Reference 3, the size of the Mg matrix is at least 50 μm since the alloys are produced by a casting method. Therefore, this reference does not show that the alloy exhibit high strength/high toughness properties on the same level as or higher than that of the above-mentioned, rare earth element-added (Mg—Zn-RE) alloys. In addition, it would involve technical difficulties (see FIGS. 1 and 2).
Patent Reference 1: JP-A 2002-309332
Patent Reference 2: JP-A 2005-113234
Patent Reference 3: JP-A 2005-113235
Patent Reference 4: Japanese Patent Application No. 2006-211523
Patent Reference 5: Japanese Patent Application No. 2007-238620
Non-Patent Reference 1: G. Bergman, J. Waugh, L. Pauling: Acta Cryst. (1957) 10 254
Non-Patent Reference 2: T. Rajasekharan, D. Akhtar, R. Gopalan, K. Muraleedharan: Nature (1986) 322 528
Non-Patent Reference 3: L. Bourgeois, C. L. Mendis, B. C. Muddle, J. F. Nie: Philo. Mag. Lett. (2001) 81 709